1. Field of the Invention
The present invention relates to cell search for mobile wireless systems, and more particularly, to a method of performing cell search and integer part frequency offset estimation for a wireless communications system.
2. Description of the Prior Art
Orthogonal frequency division multiple access (OFDMA) has become one of the most promising technologies in modern wireless communications systems, owing to its robustness against frequency-selective channels, its flexibility for multi-rate transmission considering multiple users, and so on. It is also adopted as one of the air interfaces in IEEE 802.16, known as WirelessMAN-OFDMA.
WirelessMAN-OFDMA is a connection-oriented network, in which each frame has a preamble, a downlink subframe, and an uplink subframe, for time division duplex (TDD) mode operation. The preamble is an OFDM symbol with a cyclic prefix (CP) extension like other OFDM symbols within the frame. The difference between the preamble and normal OFDM symbols is that the preamble is binary-phase shift keying (BPSK) modulated by 114 possible pseudo-noise (PN) sequences transmitted by base stations (BSs). Mobile stations (MSs) detect the transmitted PN sequence among the 114 possibilities, so that the basic information of the BS, such as its segment index and cell number, may be acquired to perform demodulation of the downlink subframes. The procedure used to detect the PN sequence is called “cell search.”
Although the selected set of PN sequences is characterized by low cross correlation, presence of cross correlation values can not be ignored when detecting the employed PN sequence, and therefore the computation power required to perform cell search is high. In addition, when considering the presence of integer part frequency offset, the uncertainty increases, such that the computation burden becomes even heavier.